PAR-006: necpp-style embeddability and diagnostics (Phase 3)

Source: docs/backlog.md (PAR-006)

Owner: Core APIs
Target: Phase 3

Resolution criteria:
- stable automation API surface documented
- binding strategy decision recorded
- geometry diagnostics catch known invalid/fragile model classes with actionable errors

Notes:
Tracks PRT-007 / CP-006 / CP-007 alignment.